Optical signals are transmitted at unique wavelengths referred to as channels. The spacing between channels is often as little as one nanometer in wavelength, so optical routing devices that combine or separate the different wavelength signals must be sensitive to such small differences in wavelength. Precisely designed devices are required to transmit the different wavelength signals with high efficiency and low crosstalk between adjacent channels.
However, it is common for the signals to drift slightly from their intended wavelength, particularly at their source. Unless more crosstalk can be tolerated, the transmission efficiency of the drifted signals is often significantly reduced. In addition, the transmission characteristics of routing devices themselves can vary during their manufacture or use.
Devices that combine or separate the different wavelength signals are referred to as multiplexers and demultiplexers, respectively. Often, the only difference between these devices is the direction of light travel through them. Multiplexers route different optical signals traveling separately in individual pathways into a common pathway. Demultiplexers route the optical signals traveling together in the common pathway back into the individual pathways.
Within multiplexers and demultiplexers, two optical mechanisms are used for routing the optical signals between the common and individual pathways--dispersion and focusing. Dispersion angularly distinguishes the different wavelength signals, and focusing converts the angularly distinguished signals into spatially distinguished signals.
For example, a focusing mechanism can be arranged to form discrete images of the common pathway in each wavelength of the different optical signals. The dispersing mechanism relatively displaces the images along a focal line by an amount that varies with the wavelength of the different signals. The individual pathways are arrayed along the focal line in positions corresponding to the displaced images of the different wavelength signals. Thus, each different wavelength signal forms a discrete image of the common pathway in a different position along the focal line, and the individual pathways are located along the focal line coincident with the image positions of the different wavelength signals.
The light energy within the common and individual pathways is distributed throughout a plane transverse to its direction of travel in a pattern defined by a mode field. Generally, the light amplitude distribution within each mode field is Gaussian. Maximum coupling efficiency occurs when the central amplitude of the imaged common pathway is exactly aligned with the central amplitudes of the respective individual pathways. Any drift in the wavelength of the different wavelength signals misaligns the central amplitudes of the paired mode fields and reduces coupling efficiency.
Spectral response curves measure coupling efficiency in units of decibel loss over a domain of wavelengths. Some small variation in decibels (e.g., one to three decibels) can generally be accommodated, and the corresponding range of wavelengths defines channel bandwidth. My copending U.S. patent application Ser. No. 08/581,186, filed Dec. 29, 1995, now U.S. Pat. No. 5,675,675, and entitled BANDWIDTH-ADJUSTED WAVELENGTH DEMULTIPLEXER, demonstrates possibilities for a tradeoff between channel bandwidth and crosstalk attenuation. A radius of the mode fields, defined at 1/e.sup.2 of the central light intensity, can be increased to enlarge the bandwidth at a cost of less crosstalk attenuation. Thus, any excess crosstalk attenuation in a design can be converted into larger bandwidths.
The ideal shape of the spectral response curve is a rectangular form resembling an inverted tophat. The bottom of the response curve is preferably as flat as possible to minimize decibel variations within the bandwidth, and the sides are as steep as possible to maximize the size of the bandwidth while maintaining the desired crosstalk attenuation in adjoining channels.
U.S. Pat. No. 5,412,744 to Dragone discloses a wavelength routing device operable as a multiplexer or demultiplexer with flattened response curves. Confocal star couplers connect two groups of waveguides (pathways) to opposite ends of a phase array. The focusing function is performed by the star couplers, and the dispersing function is performed by the phase array. The flattened response is achieved by using Y-shaped connectors to join remote ends of adjacent waveguides. Light is collected from two adjacent mode fields, and their overlapping response curves are combined.
However, additional spacing is required between pairs of adjacent waveguides to maintain the desired level of crosstalk attenuation. In comparison to similar devices without Y-shaped couplers, only one of every three waveguides can be used to avoid excessive crosstalk. This greatly diminishes the number of different wavelength signals that can be routed through the device.
A paper entitled APhased-array wavelength demultiplexer with flattened wavelength response@ by M. R. Amersfoort et al., published in ELECTRONIC LETTERS, Vol. 30, No. 4, Feb. 17, 1994, substitutes multimode waveguides for single mode waveguides in an output array to flatten spectral response. While it is possible to connect detectors to the multimode output waveguides, the device cannot be used to route different wavelength signals within a single mode optical network.
Another paper entitled Mrrayed-waveguide grating multiplexer with flat spectral response@ by K. Okamoto and H. Yamada, published in OPTICS LETTERS, Vol. 20, No. 1, Jan. 1, 1995, discloses modifications to a phase array for producing a near flat spectral response in a multiplexer. However, the path length variations required to accomplish the improved response are difficult to implement.